This is a very typical (text-book) example of an active Twin-T notch filter with adjustable Q.
However, after having posted THIS question, one of the commenters pointed out that there is a design that completely leaves out the OpAmp (OA) indicated, as it doesn't seem to serve a clear function/purpose.
I'm currently working on a design involving a high-precision Instrument Amplifier (IA), that is chained through a bunch of filters. The first one being the 60 Hz notch filter shown above, which is then output to an active 100 Hz LPF. The LPF consist of a typical
2nd Order Sallen-Key LPF, followed by a passive 1st Order RC LPF.
All the designs I have seen, are basically using this text-book Twin-T design with 1 or 2 OA's, for output and low impedance feedback, respectively. But are all these designs just a copy-paste artifact of people not thinking by themselves or are there important reasons for having the (indicated) OA in the circuit?
I have simulated the above (original) circuit here:
and and then again the single OA solution here:
This design show a pretty good response of
-30dB @ 61.4 Hz (slightly off from original) with
-3dB at ~65.6 Hz when using an 11kΩ pot set at 381Ω. But the original design seem to have better accuracy for \$f_c\$.
Q: What are the effects of removing the OA from the circuit?
(What side effects will this have on i/o impedance and component value sensitivity etc? Other pros/cons?)